1. Field of the Invention
The present invention relates to a flow rate sensor for outputting a signal in response to a flow rate of a fluid being measured, and relates to a flow rate sensor suitable for measuring an intake air flow rate of an internal combustion engine in an automobile, for example.
2. Description of the Related Art
Generally, in an automotive engine, etc., an air-fuel mixture including fuel and intake air is burnt in a combustion chamber in the engine body, and rotational output from the engine is extracted from the resulting combustion pressure, requiring that the intake air flow rate be detected in order to calculate the injection rate, etc., of the fuel with high precision.
The flow rate sensor shown in Japanese Patent Laid-Open No. 2000-2572, for example, is known as a conventional technique of this kind.
FIG. 12 is a longitudinal section showing a conventional flow rate sensor such as described in Japanese Patent Laid-Open No. 2000-2572, for example, mounted to a main passage, FIG. 13 is a partial perspective showing an assembly process for the conventional flow rate sensor, and FIG. 14 is an enlarged partial cross section of the conventional flow rate sensor in FIG. 12.
In the figures, a main passage 1 is formed into a cylindrical shape from, for example, a resin material, a metal material, etc., a small-diameter cylindrical mounting aperture 2 being disposed so as to project radially outward, and a passage forming body 3 having a vertically-aligned rectangular body being disposed so as to project radially inward from an inner wall surface of the main passage 1. A bypass passage 4 is formed into a general U shape inside this passage forming body 3, an inflow aperture 5 of the bypass passage 4 opening onto the vicinity of the axial center of the main passage 1 on a front surface of the passage forming body 3, and an outflow aperture 6 of the bypass passage 4 opening onto the main passage 1 on a lower surface of the passage forming body 3. In addition, an element insertion aperture 7 is formed in the passage forming body 3 at a position opposite the mounting aperture 2.
A flow rate sensor 10 is constituted by a casing 11, a mount plate 18, a circuit board 21, a flow rate detecting element 23, etc.
The casing 11 is formed into a stepped cylindrical shape from a resin material, for example, and is constituted by: a collar-shaped mount portion 12 formed on a base end portion of the casing; a circuit accommodating portion 13 formed into a generally rectangular overall box shape and disposed so as to extend to a first side of the mount portion 12; and a connector portion 14 formed on a second side of the mount portion 12, the connector portion sending and receiving signals to and from an external portion. A circuit board mount recess portion 15 surrounded by a peripheral wall 15a forming a rectangular shape, a mount plate interfitting groove 16 formed by cutting away a portion of the peripheral wall 15a at an extremity of the casing 11, and interfitting apertures 17 formed so as to be positioned on first and second sides of the mount plate interfitting groove 16 are disposed in the circuit accommodating portion 13.
The mount plate 18 is formed into a plate-shaped body from a metal material, for example, being composed of: a circuit board mount portion 19 formed by bending edge portions of the mount plate 18 on the left and right in FIG. 12; and an element mount portion 20 formed integrally at an extremity of the circuit board mount portion 19. A rectangular element accommodating recess portion 20a for accommodating the flow rate detecting element 23 is formed in this element mount portion 20. This mount plate 18 is mounted to the casing 11 by housing the circuit board mount portion 19 inside the circuit board mount recess portion 15 such that the element mount portion 20 fits into the mount plate interfitting groove 16. Here, an extremity of the element mount portion 20 projects from the casing 11.
The circuit board 21 is disposed on the circuit board mount portion 19, electronic components for sending and receiving electric signals to and from the flow rate detecting element 23 being mounted to the circuit board 21. First circuit board terminals 21a of the circuit board 21 and connector terminals 14a of the connector portion 14 are each electrically connected by first bonding wires 22a. 
The flow rate detecting element 23, as shown in FIG. 13, is provided with: a rectangular silicon substrate 24; a heater resistor 25 formed on a surface of the silicon substrate 24; a pair of resistance thermometers 26 formed on the surface of the silicon substrate 24 so as to be positioned to the left and right of the heater resistor 25; and a temperature-compensating resistor 27 formed on the surface of the silicon substrate 24, the flow rate detecting element 23 being disposed inside the element accommodating recess portion 20a. Second circuit board terminals 21b of the circuit board 21 and element terminals 23a of the flow rate detecting element 23 are each electrically connected by second bonding wires 22b. 
Moreover, the heater resistor 25, the resistance thermometers 26, and the temperature-compensating resistor 27 are electrically connected to each of the element terminals 23a by a wiring pattern (not shown) formed on the surface of the silicon substrate 24. Furthermore, the electronic components mounted to the circuit board 21 constitute a heater control circuit for controlling the heater resistor 25 of the flow rate detecting element 23, an amplifying circuit for amplifying detection signals from each of the resistance thermometers 26, a reverse-current sensing circuit, etc.
A stopper member 28 is constituted by a stopper main body 29 and an elastic protrusion 30. The stopper main body 29, as shown in FIG. 13, is formed by: an elongated plate portion 29a extending flatly so as to lie across the mount plate interfitting groove 16; interfitting protrusions 29b positioned on left and right sides of the elongated plate portion 29a so as to project toward the interfitting apertures 17 of the circuit accommodating portion 13 and fit into the interfitting apertures 17; a central protrusion 29c positioned between the interfitting protrusions 29b so as to fit into the mount plate interfitting groove 16 and, as shown in FIG. 14, extend to a position in proximity to the second bonding wires 22b; and a recess portion 29d formed between the elongated plate portion 29a and the central protrusion 29c. The elastic protrusion 30 is composed of a flexible elastic material such as silicone rubber, for example, and is fixed to a leading edge portion of the central protrusion 29c. The stopper member 28 is mounted to the casing 11 such that the interfitting protrusions 29b fit into the interfitting apertures 17. Here, the elastic protrusion 30, as shown in FIG. 14, is placed in contact with a surface of the flow rate detecting element 21 in an elastically-deformed state.
A sealant 31 is formed from a silicone gel, for example, and is injected inside circuit board mount recess portion 15, as shown in FIG. 14, so as to cover the surface of the circuit board 21, the bonding wires 22a and 22b, and the connector and element terminals 14a and 23a. Thus, short-circuiting of the bonding wires 22a and 22b is prevented and the electronic components mounted to the circuit board 21 are protected.
A cover body 32 is mounted to the casing 11 such that a peripheral portion thereof is fixed by an adhesive to the peripheral wall 15a of the circuit board mount recess portion 15 and the stopper main body 29 so as to leave space between the front surface of the sealant 31 and the cover body 32. Thus, the circuit board mount recess portion 15 is sealed over, and the stopper member 28 is held with the elastic protrusion 30 placed in contact with the surface of the flow rate detecting element 21 in an elastically-deformed state.
The flow rate sensor 10 constructed in this manner is plugged into the main passage 1 by mounting an O ring 8 to the root end (the mount portion 12 end) of the circuit accommodating portion 13, inserting the circuit accommodating portion 13 so as to project inside the main passage 1 from the mounting aperture 2, and fastening the mount portion 12 to the fixing seat 2a of the mounting aperture 2 securely by a fixing screw 33. At this time, the element mount portion 20 of the flow rate sensor 10 is inserted inside the element insertion aperture 7 and the flow rate detecting element 23 is disposed inside the bypass passage 4. The flow rate sensor 10 is mounted to the main passage 1 airtightly by disposing the O ring 8 in a compressed state between the mounting aperture 2 and the circuit accommodating portion 13.
This main passage 1 is connected partway along an intake air line of the engine, an intake air filtration apparatus (not shown) being connected to a first end thereof, and an intake air manifold communicating with the inside of cylinders of the engine (not shown) being connected by means of a throttle valve, etc., (not shown) to a second end. Air cleaned by the intake air filtration apparatus flows through the inside of the main passage 1 from right to left in FIG. 12, is directed inside the bypass passage 3 through the inflow aperture 5, flows over the surface of the flow rate detecting element 23 (the silicon substrate 24), then flows out into the main passage 1 through the outflow aperture 6.
A heating current which flows through the heater resistor 25 is controlled by a circuit constructed on the circuit board 21 such that the average temperature of the heater resistor 25 is higher than the temperature of air detected by the temperature-compensating resistor 27 by a predetermined amount. Thus, the flow rate of the air is detected by making use of the cooling effect the flow of air exerts on the heater resistor 25 and changes in the resistance values of each of the resistance thermometers 26.
If the conventional flow rate sensor 10 constructed in this manner is used as an intake air flow rate detecting apparatus in an internal combustion engine, for example, it is normally plugged in immediately downstream from the intake air filtration device. This intake air filtration apparatus is normally fastened to a vehicle body or chassis inside an engine compartment. Thus, vibrational acceleration has been comparatively small since vibrations to which the flow rate sensor 10 is subjected are transmitted through the vehicle body or the chassis.
However, in recent years, with demand for reductions in the size of engine compartments, intake air filtration apparatuses are increasingly being installed immediately above the engine and fastened to the engine. The flow rate sensor 10 may also be fastened to a throttle body and then the throttle body is fastened directly onto the engine. In such cases, since the vibrational acceleration induced by operation of the engine is transferred to the flow rate sensor 10 directly through the intake air filtration apparatus, vibrational acceleration occurring in the flow rate sensor 10 is extremely large compared to cases where the intake air filtration apparatus to which the flow rate sensor 10 is fastened is fastened to the vehicle body or the chassis.
Because the conventional flow rate sensor 10 is securely fastened to the fixing seat 2a of the main passage 1 by the fixing screw 33, when the flow rate sensor 10 is subjected to vibration, the vibrational mode is one of cantilever support in which the fixing seat 2a and the O ring 8 constitute a fixed end and the element mount portion 20 constitutes a free end. Thus, vibrational acceleration occurring in the circuit board 21, the bonding wires 22b, and the sealant 31 is larger than vibrational acceleration occurring at the fixing seat 2a. 
At the same time, a soft silicone gel, etc., is normally used for the sealant 31 in order to improve heat shock tolerance. The volume of silicone gel (sealant 31) is large in order to seal the entire front surface of the circuit board 21. In other words, a large volume of silicone gel (sealant 31) is formed into a single elastic body. Because the cover body 32 is mounted to the front surface of the silicone gel (the sealant 31) so as to leave space, there is no structure mechanically restraining the silicone gel (the sealant 31). Consequently, if the sealant 31 is viewed as a single elastic body, the characteristic frequency of the sealant 31 is extremely low compared with the characteristic frequency of the casing 11 in which the circuit board 21 and the mount plate 18 are installed. Because of this, the circuit board 21 tries to vibrate in a vibrational mode similar to the vibration to which the flow rate sensor 10 is subjected, but portions excluding an electrical connection portion 34 between the terminals 21b and 23a of the bonding wires 22b try to vibrate together with the sealant 31 in a vibrational mode differing from that of the circuit board 21. Thus, stress corresponding to displacement due to the differences in the vibrational modes arises in concentration at the interface between the sealant 31 and the circuit board 21, in other words, at the electrical connection portion 34.
Thus, when stress equal to or greater than the bond strength of the electrical connection portion 34 of the bonding wires 22b disposed inside the sealant 31 arises, separation from the electrical connection portion 34 or breakage of the bonding wires 22b arises, bringing about abnormalities in the output from the flow rate sensor 10, thereby giving rise to problems.
One countermeasure that may be considered in order to solve problems of this kind is to change the material of the sealant 31 to an epoxy resin, for example, to increase the overall rigidity and hardness of the sealant 31. In that case, problems such as those described above can be solved because the circuit board 21, the bonding wires 22b, and the sealant 31 vibrate together. However, if the environment in which the flow rate sensor 10 is mounted is such that the flow rate sensor 10 is exposed to xe2x80x9cthermal shockxe2x80x9d, for example, in which high ambient temperatures and low ambient temperatures repeatedly alternate, because the highly-rigid epoxy resin is in close contact with the electrical connection portion 34, the epoxy resin and the bonding wires 22b are simultaneously subjected to repeated thermal expansion and thermal contraction due to the heat drop of the thermal shock. In such cases, a great deal of thermal stress resulting from the high rigidity of the epoxy resin is generated repeatedly in the electrical connection portion 34 due to differences in coefficients of thermal expansion between the epoxy resin and the bonding wires 22b. Thus, when thermal stress equal to or greater than the bond strength of the electrical connection portion 34 arises, separation from the electrical connection portion 34 or breakage of the bonding wires 22b arises. In addition, when thermal stress arises repeatedly in the electrical connection portion 34 and exceeds the fatigue limit of the bonding wires 22b, breakage of the bonding wires 22b occurs.
The present invention aims to solve the above problems and an object of the present invention is to provide a flow rate sensor having superior durability in which the occurrence of output anomalies is suppressed by reducing stress occurring at an electrical connection portion as a result of vibration or thermal shock to avoid wire breakage in the electrical connection portion.
With the above object in view, according to a first aspect of the present invention, there is provided a flow rate sensor including a stanchion portion in which terminal conductors are embedded; a flow rate detecting element for detecting a flow rate of a fluid being measured, the flow rate detecting element being disposed at a first end of the stanchion portion; connecting conductors for electrically connecting electrode terminals of the flow rate detecting element and end portions of the terminal conductors exposed from the stanchion portion, respectively; and an electronic circuit portion for controlling an electric current flowing to the flow rate detecting element, the electronic circuit portion being electrically connected to the flow rate detecting element by means of the terminal conductors and the connecting conductors. The flow rate sensor has a plug-in construction in which the first end of the stanchion portion is inserted into an aperture opening onto a main passage through which the fluid being measured flows so as to extend into the main passage to detect the flow rate of the fluid being measured. Electrical connection portions between the electrode terminals of the flow rate detecting element and the connecting conductors and between the end portions of the terminal conductors and the connecting conductors are sealed by a first sealant, and the first sealant is sealed by a second sealant, the second sealant having a physical property of higher hardness than the first sealant.
Therefore, the occurrence of output anomalies is suppressed by reducing stress occurring at the electrical connection portion as a result of vibration or thermal shock to avoid separation of the electrical connection portions or wire breakage in the connecting conductors, thereby providing the flow rate sensor having superior durability.
According to a second aspect of the present invention, there is provided a flow rate sensor including a stanchion portion in which terminal conductors are embedded; a flow rate detecting element for detecting a flow rate of a fluid being measured, the flow rate detecting element being disposed at a first end of the stanchion portion; connecting conductors for electrically connecting electrode terminals of the flow rate detecting element and end portions of the terminal conductors exposed from the stanchion portion, respectively; and an electronic circuit portion for controlling an electric current flowing to the flow rate detecting element, the electronic circuit portion being electrically connected to the flow rate detecting element by means of the terminal conductors and the connecting conductors. The flow rate sensor has a plug-in construction in which the first end of the stanchion portion is inserted into an aperture opening onto a main passage through which the fluid being measured flows so as to extend into the main passage to detect the flow rate of the fluid being measured. A covering member is airtightly mounted to the stanchion portion and the flow rate detecting element so as to envelop the electrode terminals of the flow rate detecting element, the end portions of the terminal conductors, and the connecting conductors, a portion of said flow rate sensor enveloped by the covering member constituting a hollow cavity.
Therefore, the occurrence of output anomalies is suppressed by reducing stress occurring at the electrical connection portions as a result of vibration or thermal shock to avoid wire breakage at the electrical connection portions, thereby providing a flow rate sensor having superior durability.